Pyridoxal phosphokinases. II. Effects of inhibitors.

Many compounds have been described that inhibit growth by interfering at one point or another in the metabolism of vitamin Bg (2). These include a variety of compounds related structurally to the vitamin, together with an array of carbonyl reagents generally known to inhibit pyridoxal phosphate enzymes. In no case has the site of action of such inhibitors in tivo been established. One of the first enzymatic loci at which such compounds might act is pyridoxal kinase, which is responsible for the conversion of the three forms of vitamin Bg to their corresponding phosphates. Hurwitz (3, 4) found that several analogues of pyridoxal that contained an unsubstituted position 6 and a free hydroxymethyl group in position 5 were phosphorylated by the pyridoxal phosphokinase of yeast and, hence, served as competitive substrates for this enzyme; other compounds inhibited the kinase without themselves being phosphorylated. Earlier we described briefly an inhibition of pyridoxal kinase by carbonyl reagents, which was attributed to the condensation products formed between these compounds and pyridoxal (5, 6). We present herein a comparison of the sensitivity of purified pyridoxal kinases from several different sources to inhibition by carbonyl reagents, by their condensation products with pyridoxal, and by a number of structural analogues of vitamin Be.